1. Field of the Invention
The present invention relates to compositions for use as thickening agents in heavy brine systems and to aqueous well servicing fluids prepared thereon. More specifically the invention relates to viscosifying heavy brine compositions with cationic polysaccharides.
2. Description of the Related Art
Thickened aqueous mediums, particularly those containing oil field brines, are commonly used as well servicing fluids such as drilling fluids, workover fluids, completion fluids, packer fluids, well treating fluids, subterranean formation treating fluids, fracturing fluids, spacer fluids, hole abandonment fluids, and other aqueous fluids in which an increase in viscosity is desired. Workover fluids are those fluids used during remedial work in a drilled well. Such remedial work includes removing tubing, replacing a pump, cleaning out sand or other deposits, logging, etc. Workover also broadly includes steps used in preparing an existing well for secondary or tertiary recovery such as polymer addition, micellar flooding, steam injection, etc. Fracturing fluids are used in oil recovery operations where subterranean is treated to create pathways for the formation fluids to be recovered.
Completion fluids are those fluids used during drilling, completion, or recompletion, of the well. Completion operation normally includes perforating the casing, setting the tubing and pumps in petroleum recovery operations. Both workover and completion fluids are used in part to control well pressure, to prevent the well from blowing out during completion or workover, or to prevent the collapse of well casing due to excessive pressure build-up.
Polymers and chemicals are added to the brines used in well servicing fluids for various reasons that include, but are not limited to, increasing viscosity, and increasing the density of the brine. For example, in order to obtain brine having a salinity of about 11.5 to 19.5 pounds per gallon (ppg), ionic salts of calcium, magnesium and zinc are often added to the brine. Water-thickening polymers serve to increase the viscosity of the brines and thus retards the migration of the brines into the formation and lifts drilled solids from the well-bore. Another advantage of using heavy brines is the ability to penetrate deeper in oil formations.
Polysaccharides such as hydroxyethyl cellulose (HEC), carboxylmethylhydroxyethyl cellulose, carboxymethyl cellulose (CMC) and synthetic polymers such as polyacrylamides (PAM) are commonly used to increase the viscosity of brines. These polymers are incapable of hydration and increasing the viscosity of heavy brines, particularly greater than 11.5 ppg. Further, viscosifying heavy brines presents problems due to incompatibility and/or precipitation of polysaccharides in the presence of dissolved multivalent salts. Use of high molecular weight synthetic polymers such as PAM provides relatively low viscosities. Moreover, when PAM emulsions are used, inverting surfactants are required, and these are usually added separately during operation involving additional process steps. Further, these polymers when used to viscosify brines that contain zinc ions in the range of 0.1 wt % to 7 wt % do not function effectively without the use of additives. As heavy brines containing high levels of zinc salts are widely used as workover fluids, drilling fluids, fracturing fluids and completion fluids in oil field operations, development of a process for viscosifying heavy brines is highly desirable.
Among the problems in using polysaccharides such as HEC in thickening is the formation of clumps referred to as “fish eyes” due to poor hydration that can cause clogging and formation damage. HEC and other polysaccharides surface-hydrate too rapidly and non-uniformly, resulting in clumps. Many of these polysaccharides hydrate only after elevating the temperature of the brine and when these are normally supplied dry powders require special preparation and/or mixing and dispersing equipment when added to brines. Further, exposure to higher down-hole temperatures that dissolve the clumps cause brine viscosities to fluctuate resulting in unpredictable and undesirable latent viscosities. Moreover, many of the polysaccharides also tend to form separate gummy polymer layers in high density brines, particularly in the range of densities of 11.5 to 19.5 ppg.
Various attempts have been made involving additional time and expense to overcome the dissolution problems described above. These include additional steps including treatment of the polysaccharides with additives in solvent media that increases the risk of flammability. For example, in U.S. Pat. No. 4,392,964 to R. House et. al., a polymer composition with organic solvents prepared by mixing 5-30 parts of HEC with at least 40 parts of isopropanol mixed with 3-40 parts water in order to thoroughly wet the HEC prior to addition to the brine. In U.S. Pat. No. 4,415,463 to B. Mosier et. al., natural polysaccharide gums such as carageenan and locust bean are treated with a basic nitrogen reagent to wet the polymer prior to addition to the brine. Such treatments required dry powders of the polysaccharide gums or solutions of isopropanol-water prior to contacting with alcohol and the basic nitrogen reagent to ensure wettability of the treated product. In U.S. Pat. Nos. 4,435,564 and 4,439,333 to R. House, HEC is first activated such that the HEC will disperse and hydrate in heavy brines. The activation process comprises admixing HEC in a solution of either an amino or phenolic compound and a water soluble organic liquid prior to brine addition. Some of these additives also become ineffective when added to brines having densities above about 12 ppg. In U.S. Pat. No. 4,420,406 to R. House et al. it was disclosed that heavy brine solutions having a narrow density range of 14.2 to 15.6 ppg could be effectively thickened with HEC depending on the presence or absence of calcium chloride and the specific amount of zinc bromide in the brine.
Synthetic polymers disclosed in U.S. Pat. No. 4,619,773 are thickened brines containing AMPS/vinylamide and optionally acrylamide. U.S. Pat. No. 6,346,588 to Franchel et. al., discloses terpolymers based on (meth)acrylamide, hydroxyalkyl (meth)acrylate and sulfobetaine monomers and their use as thickeners for aqueous salt solutions containing calcium chloride and/or calcium bromide and/or zinc bromide.
Given the above, it is desirable to have a natural polymer such as polysaccharide that viscosifies the heavy brines containing calcium, magnesium and zinc salts in well servicing fluids without the use of additives and/or additional treatment steps prior to mixing with the heavy brines. In particular, it is desirable to obtain a polysaccharide that viscosfies heavy brine particularly in the range of densities of 11.5 ppg to 19.5 ppg. In accordance with the present invention, there is provided a well servicing fluid composition and a process of preparation thereof comprising a heavy brine compositions containing calcium and zinc salts and a cationic polysaccharide for viscosifying the brine.